Secondary recovery method

ABSTRACT

Significant improvement in the recovery of hydrocarbons from a subterranean hydrocarbon-bearing formation containing acidsoluble components is accomplished by injecting into the formation via an injection well drilled into a formation communicating with an adjacent producing well and containing acid-soluble components which may or may not have water-sensitive clays and shales included therein, an aqueous acidic solution of a phosphate ester of prescribed formula whereupon the acid component reacts with the acid-soluble components of the formation creating passageways or enlarging existing passageways thus facilitating the flow of fluids therein and the phosphate ester prevents post precipitation of dissolved salts and thereby increases the recovery of hydrocarbons from the formation through the adjacent producing well.

United States Patent [191 Tate [ March 6, 1973 SECONDARY RECOVERY METHODJack F. Tate, Houston, Tex.

[73] Assignee: Texaco Inc., New York, N.Y.

[22] Filed: June 21, 1971 [21] Appl. No.: 155,310

[75] Inventor:

[52] U.S. Cl. ..166/271, 252/855 C, 252/855 D [51] Int. Cl. ..E21b43/22, E2lb 43/27 [58] Field of Search ..166/271, 259, 281, 282, 307;

- 252/855 B, 8.55 C, 8.55 D

[56] References Cited UNITED STATES PATENTS Primary ExaminerStephen J.Novosad Attorney-Thomas H. Whaley et al.

[5 7] ABSTRACT Significant improvement in the recovery of hydrocarbonsfrom a subterranean hydrocarbon-bearing formation containingacid-soluble components is accomplished by injecting into the formationvia an injection well drilled into a formation communicating with anadjacent producing well and containing acid-soluble components which mayor may not have water-sensitive clays and shales included therein, anaqueous acidic solution of a phosphate ester of prescribed formulawhereupon the acid component reacts with the acid-soluble components ofthe formation creating passageways or enlarging existing passagewaysthus facilitating the flow of fluids therein and the phosphate esterprevents post precipitation of dissolved salts and thereby increases therecovery of hydrocarbons from the formation through the adjacentproducing well.

7 17 Claims, No Drawings SECONDARY RECOVERY METHOD FIELD OF THEINVENTION This invention relates to a method for the recovery ofhydrocarbons from subterranean hydrocarbon-bearing formations containingacid-soluble components in which the permeability of the formationcommunicating between the producing well and adjacent injection well isincreased by treating with an aqueous acidic solution of a phosphateester of prescribed formula thereby facilitating the flow of fluidsthrough the formation resulting in increased hydrocarbon recovery viathe production well.

DESCRIPTION OF THE PRIOR ART In recovering oil from oil-bearingreservoirs it usually is possible to recover only a minor part of theoriginal oil in place by the primary recovery methods which utilize thenatural forces present in the reservoir. As a result, a variety ofsupplemental recovery techniques have been utilized to increase therecovery of oil from subterranean hydrocarbon-bearing reservoirs orformations. Although these supplemental techniques are commonly referredto as secondary recovery operations in fact they may be primary ortertiary in sequence of employment. In such techniques, a fluid isintroduced into the formation in order to displace the oil therein to asuitable production system through which the oil may be withdrawn to thesurface of the earth. Examples of displacing media include gas, aqueousliquids such as fresh water or brine, oil-miscible liquids such asbutane, or a water and oil-miscible liquid such as an alcohol.Generally, the most promising of the secondary recovery techniques isconcerned with the injection into the formation of an aqueous floodingmedium either alone or in combination with other fluids.

In the application of these conventional procedures for the productionof hydrocarbons from similar formations by the secondary recovery methodof water-injection, one of the principal difficulties that has beenencountered is the generally low production response realized because ofthe low permeabilities and the consequent low rate of water acceptanceof the communicating formation. Thus, these unfavorably low responsesboth in injection rate and in overall production have led to theabandonment of hydrocarbon production by water-injection methods frommany carbonate formations after only a minimal amount of theoil-in-place has been produced.

One of the remedial measures that has been used frequently to increasewater-injectivity in formations containing acid-soluble components isacid-treating of injection wells to improve the permeability surroundingthe injection well bore, and thereby increasing the flow capabilities ofthe formation in the vicinity of the injection well bore. Thesemeasures, however, may result in only a temporary response in productionimprovement.

In acidizing an injection well utilizing the commonly employed procedurea non-oxidizing mineral acid, such as hydrochloric acid, is introducedinto the injection well, and through the application of sufficientpressure is forced into the adjacent formation, where it reacts with theacid-soluble components, particularly the carbonates to dissolve them toform the respective salt of the acid, carbon dioxide and water, therebyincreasing the permeability of the formation adjacent the bore of theinjection well. The technique is not limited to application informations of high acid solubility. Sandstone and gypsum-containingformations may require acidization of the injected water is unstablewith respect 'to CaCO Fe(OI-I) or other salts.

If desired, the acidization may be carried out at an injection pressuresufficiently great to create fractures in the strata or formation whichhas the desired advantage of opening up passageways into the formationalong which the acid can travel to more remote areas from.

the injection well bore. Conversely, fracturing a formation into which adisplacing fluid is injected quite often is not desirable since sweepefficiency may be decreased. The salts formed upon neutralization of theacid are extensively water soluble and may pass through the formationdissolved in the displacing fluid.

There are, however, troublesome complications attending the use ofhydrochloric acid, or other similar non-oxidizing mineral acids. In theacidizing process, the following primary beneficial reaction occurs:CaCO 2I-ICl CaCl E 0 C0 1. Under the higher pressures required toconduct an acidization, the CO is dissolved in the reaction mixtureconsisting of spent acid and commate water: CO H H CO I-I I-ICO,,' 2H COThe equilibria may be summarized and written:

(maroon: CHCOI H1901,

mo coil,

After acidization is complete, the well is often returned directly toinjection, the reaction products of the dissolution being forced intothe formation in a solution of the displacing fluid. As the fluid movesaway from the well bore, pressure diminishes allowing dissolved gasessuch as CO to break out of solution, inducing insoluble carbonates, suchas CaCO to postprecipitate.

Similarly, acid-dissolved iron salts may reprecipitate within theformation as hydrous iron oxides a distance from the well bore when pHrises due to exhaustion of the acid. In like manner, gypsum mayprecipitate if the injected fluid contains sulfate ion and if itstemperature increases within the formation since the gypsum has aninverse solubility relationship. Precipitation of this type when itoccurs within the capillaries of a tight formation can severely lessenthe injection rate by plugging such capillaries.

It is known that molecularly dehydrated polyphosphates, are effective inretarding CaCO precipitation. These polyphosphates are unsatisfactory inthe method of the present invention because they undergo rapidhydrolysis in the presence of the mineral acid components. As a result,the scale inhibiting properties of these polyphosphates are destroyed.In addition, one hydrolytic reaction product, the phosphate ion (POf),can precipitate with calcium +2 or barium ions present in the producedwater, causing additional plugging or scale deposition and they furtheraggravate the problem. The so-called glassy" phosphates are known scaleinhibitors. However, these glassy phosphates are unsatisfactory becauseof their slight solubility in acidic media and the tendency to formobjectionable hydrolytic reaction products.

It is also known to employ various organic polymers to prevent theprecipitation of the mineral salts. Many of these polymeric materialsare unstable in mineral acids. In such acidic media they undergospontaneous depolymerization to an ineffective species. A representativepolymeric material which undergoes such hydrolysis in the presence ofacids is polyacrylamide. In addition, this polymer has a furtherdisadvantage in that it is unstable in aqueous media at temperatures ofabout 300 F. and upwards. Many wells that are being treated by themethod of the present invention have bottom hole temperatures of 250300F. or higher.

The chemically altered natural polymers and natural polymers themselves,are effective inhibitors to prevent the precipitation of mineral salts.However, many of these materials such as sodium carboxymethylcelluloseprecipitate or decompose in the presence of mineral acids. Other knownsequestering agents such as citric or tartaric acids, and/or complexingagents such as ethylenediaminetetraacetic acid and its watersolublesalts are known inhibitors to prevent the deposition of boiler scale inaqueous media. However, such materials are not applicable in the methodof the present invention because they are not appreciably surface activeand do not adsorb on the formation face.

The primary object of the present invention is to provide a method forthe improved recovery of hydrocarbon fluids from subterraneanfluid-bearing formations wherein a composition comprising an aqueousacidic solution of a phosphate ester of prescribed formula is injectedinto a formation communicating between a producing well and an adjacentinjection well, said formation containing acid-soluble components and insome instances also containing water-sensitive clays or shales, andwhereafter the acid component of the said composition reacts with theacid-soluble components of the formation to increase permeability of theformation thereby facilitating the flow of fluids therethrough and thephosphate ester component prevents the postprecipitation of compoundsformed by the reaction of the acidic component with the formation.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a new and improved method for the recovery of hydrocarbonsfrom subterranean hydrocarbon-bearing formations in which there isinjected into the formation via an injection well drilled into aformation communicating with an adjacent producing well and containingacid-soluble components which may or may not have water-sensitive claysincluded therein, a composition comprising an aqueous acidic solution ofa phosphate ester of prescribed type which is capable of reacting withthe acid-soluble components of the formation to increase thepermeability and porosity thereof and the phosphate ester componentprevents precipitation of compounds formed by the said reaction of theacid component thereby permitting a sub stantial increase of productionof hydrocarbons from the formation via the production well.

An advantage resulting in treating subterranean hydrocarbon-bearingformations having acid-soluble components therein with the aqueousacidic solution of the phosphate ester is that the post-precipitation ofacid-dissolved salts is prevented or materially decreased. Suchpost-precipitation, whether it be of carbonates from evolution of CO:from solution, hydrous iron oxides from a rise in pH, or of gypsum froman increase in formation temperature, can lead to plugging of formationcapillaries and a consequent reduction of permeability. Such an eventcan reduce injectivity and accordingly the rate of production fromadjacent wells.

DESCRIPTION OF THE INVENTION The method of the present invention in itsbroadest embodiment comprises introducing via an injection well drilledinto a hydrocarbon-bearing formation containing acid-soluble componentsand communicating with a producing well a fluid composition comprisingan aqueous acidic solution of a phosphate ester of prescribed formula inamounts sufficient to react with the formation so as to increasesubstantially the flow capability of the formation and to thereafterproduce hydrocarbons from the said subterranean formation at anincreased rate through the production well.

The phosphate ester useful in preparing the aqueous acidic compositionsof the present invention has the general formula 1 1 III wherein R iseither hydrogen or a CI3HZ7O (CZH4O)III C2H4 group in which n is 1 to 3,m is equal to 2 or 3, and R is a hydrogen atom not more than m minus onetimes, or a mixture of said esters, said ester having a molecular weightin the range of from about 750 to about 1730.

The phosphate esters in the composition of the present invention can beprepared in a known manner such as for example by ethoxylation oftridecanol and subsequent phosphorylation of the ethoxylated tridecanol.

The corresponding salts of the ester can be prepared by neutralizationof the ester with caustic soda or potassium hydroxide.

A preferred ester is an ester having the structural formula l owin 2which has the empirical formula C82H l70 2S 4 The admixtures of estersmay contain from about 30 to 70 percent by weight of the C l-l o l esterand about 70 to 30 percent by weight of the c u o r, ester. A preferredrange is about 40 to 60 percent of the C ester and about 60 to 40percent of the C ester.

Preferably, the acidic aqueous treating composition of this inventioninjected into the hydrocarbon-bearing formation comprises an aqueoussolution of from about 3 to about 30 percent, preferably 5 to 28 percentby weight of a non-oxidizing mineral acid, such as hydrochloric acid,etc., which contains dissolved therein between about 0.1 to about 2.5percent by weight, preferably 0.3 to 1.5 percent based on the totalsolution weight of the phosphate ester.

The aqueous acidic solutions employed in the method of this invention,preferably contain an inhibitor to prevent or greatly reduce corrosionattack on metalswA variety of such inhibitors are known in the art,e.g., certain compounds of arsenic, nitrogen or sulfur as described byGrebe et al. in U.S. Pat. No. 1,877,504. Likewise, resin-amine typeinhibitors, as illustrated in U.S. Pat. No. 2,758,970, may be utilized.A small but effective amount of the inhibitor is employed whichgenerally ranges from about 0.02 percent to about 1.5 percent by weightof the aqueous solution.

In the first step of preparing the aqueous acidic composition of thisinvention, a solution containing from about 3 to about 30 percent byweight of a non-oxidizing mineral acid, such as hydrochloric acid, inwater is prepared. An inhibitor to prevent corrosion on the metalequipment associated with the wells is usually added with mixing in thenext step. The required amount of the phosphate ester is then admixedwith the aqueous acidic solution.

The method of this invention can be carried out with a wide variety ofinjection and production systems which will comprise one or more wellspenetrating the producing strata or formation. Su'ch wells may belocated and spaced in a variety of patterns which are well-known tothose skilled in the art. For example, the so-called line flood patternmay be used, in which case the injection and producing systems arecomposed of rows of wells spaced from one another. The recovery zone,i.e., that portion of the producing formation from which hydrocarbonsare displaced by the drive fluid to the production system, in thisinstance will be that part of the formation underlying the area betweenthe spaced rows. Another pattern which is frequently used is theso-called circular flood in which the injection system comprises acentral injection well while the production system comprises a pluralityof production wells spaced about the injection well. Likewise, theinjection and production systems each may consist of only a single welland here the recovery zone will be that part of the producing strataunderlying an elliptical-like area between the two wells which issubject to the displacing action of the aqueous drive fluid. For a moreelaborate description of such recovery patterns reference is made toUren, L.C., Petroleum Production Engineering-Oil Field Exploitation,Second Edition,

McGraw Hill Book Company, Inc., New York, 1939, and to US. Pat. Nos.3,472,318 and 3,476,182.

In carrying out the method of this invention, the aqueous acidicsolution of the phosphate ester is forced, usually via a suitablepumping system, down the well bore of an injection well and into theproducing formation through which it is then displaced together withhydrocarbons of the formation in the direction of a production well. Asthose skilled in the art will readily understand, the pressure employedis determined by the nature of the formation, viscosity of the fluid,and other operating variables. The acidization method of this inventionmay be carried out at a pressure sufficient merely to penetrate theformation or it may be of sufficient magnitude to overcome the weight ofthe overburden and create fractures in the formation. Propping agents,to prop open the fractures as created, for example 20 to 60 mesh sand,in accordance with known fracturing procedures, may be employed inadmixture with the aqueous acidic solution containing the citedphosphate ester.

The formation may be treated continuously with composition or suchtreatment may be temporary. If desired, however, after a time,conventional flooding may be resumed. The aqueous acidic solution of thephosphate ester also may be applied in a modified water flood operationin which there is first injected into the well bore a slug of theaqueous acidic solution of phosphate ester which is forced underpressure into the subterranean formation. This first step is thenfollowed by a similar injection step wherein a slug of an aqueous drivefluid, such as water, is injected, which is thereafter followed by arepetition of the two steps. This sequence may be repeated to give acontinuous cyclic process. The size of the slugs may be varied withinrather wide limits and will depend on a number of conditions, includingthe thickness of the formation, its characteristics and the conditionsfor the subsequent injection of the aqueous drive medium.

In the method of this invention, the phosphate ester component of thecomposition provides means whereby ions produced by the reaction of theacid component with the formation having tendencies to precipitate assalts such as CaCO hydrous iron oxide and CaSO '2H O combine with thephosphoric acid moiety to form a highly stable complex therewith so thatsolid calcium carbonate does not precipitate from the spent treatingsolution. This binding up of the aforementioned calcium ions from weaklyionizable compounds permits the formed ion-phosphoric acid ester complexto remain dissolved in the treating solution and pass through theformation pores. Further, the phosphate ester component of thecomposition provides means whereby the nucleation and growth of thesolid itself is thwarted, so that solid salts do not precipitate fromthe spent treating solution. Finally, the phosphate ester component ofthe composition provides means whereby continuous protection againstpost-precipitation of salts is obtained for a considerable period oftime subsequent to treatment due to continuous slow desorption of thecomponent from the formation faces. In contrast, use of surfactantshaving merely dispersant and suspending properties and not possessingthe capability of molecularly binding up these produced ions orthwarting the nucleation and growth of solid salts thereof will permitpost-precipitation of said salts from such treating solution with thelikelihood of plugging up the formation passageways during subsequentrecovery of desirable formation hydrocarbons therethrough.

It should be understood that the concentration of the phosphate esterand the acid components of the composition are chosen to provide adisplacing fluid of the desired rhelogical properties. Similarly, theappropriate phosphate ester is selected on the basis of the formationbeing treated as well as other operating conditions employed.

EXAMPLE 1 Through a water injection well drilled into a limestoneformation there is displaced under pressure down the tubing and into theformation an aqueous acidic solution containing 1 percent by weightbased on the total weight of the solution of a phosphate ester soldunder the trade name Phosphonol T, dissolved in a 15 percent by weightaqueous solution of hydrochloric acid. The pressure required to injectthe required volume of water declines considerably and no increase insaid pressure is noted subsequent to treatment, indicating that postprecipition of CaCO within the formation leading to permeabilityreduction is prevented or materially lessened. The well is then returnedto conventional water injection. After about 30 days the production ofhydrocarbons from an adjacent producing well is substantially increased.

The ester employed is a mixture of esters of polyphosphonic acidscontaining polyethoxy linkages partially esterified withalkoxypolyethoxyethanols. Such mixed esters on typical analysis analyzed51.9 percent carbon, 9.7 percent hydrogen, 8.8 percent phosphorous and29.6 percent oxygen (by difference). The apparent empirical formula is CI-l P O and the mixture has a calculated molecular weight of 965. Theneutralization number was found to be 253. NMR analysis showed themixture to comprise highly branched alkylated ethoxylated acidphosphates containing a 5:1 ratio of ortho and pyro-phosphates. Theratio of ethylene oxide radical (CI-I --CH -O), tridecyl radical (C I-Iand phosphate ester radical CH OP=O) are 311:1, respectively.

EXAMPLE II A flooding operation is carried out in an oil-containingreservoir in accordance with the process of this invention. Fourinjection wells are arranged in a rectangular pattern around a singlecentrally located production well in this system. A slug consisting of75 barrels of an aqueous acidic solution containing 0.8 percent byweight, based on the total weight of the solution, of the same phosphateester as Example I, in a 15 percent by weight aqueous solution ofhydrochloric acid, is displaced via each of the four injection wellsinto the formation at a rate of about 50 bbl/day. In the next step, 100barrels of water are injected under pressure into the producingformation through each injection well at a rate of about 55 bbl/day.This sequence of operations is repeated numerous times and the result isan increased injection rate of the drive streams into the injectionwells and a subsequent increase in the rate of production ofhydrocarbons via the production well.

EXAMPLE [II An injection well in a formation containing about 30% I-IClsoluble material is treated with 500 gallons of conventional 15% I-ICIfollowed by 1500 gallons of 15% HC] containing 1% by weight of the samephosphate ester of Example I. The aqueous acidic phosphate estercomposition is displaced from the tubing into the formation with leasewater and the well shut in for 24 hours. Thereafter the well is returnedto water injection. The injectivity of the well is materially increasedfor a sustained period of time resulting in enhanced hydrocarbonrecovery.

Obviously, many modifications and variations of the inventions ashereinabove set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. A method for the recovery of hydrocarbons from a hydrocarbon-bearingformation containing acid-soluble components having at least oneinjection well and at least one production well penetrating the saidformation and in fluid communication, which comprises displacing throughthe formation a composition comprising an aqueous acidic solution of aphosphate ester containing 3 to 30 percent by weight of a non-oxidizingmineral acid and 0.1 to 2.5 percent by weight of said phosphate ester,and recovering hydrocarbons through the production well, said phosphateester having the general formula II 0 (1pm,, or ()A wherein R is eitherhydrogen or a C, I-I O(C H,,O), ,C H group in which n is l to 3, m isequal to 2 or 3 and R is a hydrogen atom not more than m minus onetimes, A is hydrogen or an alkali metal, said ester having a molecularweight in the range of from about 750 to about 1730.

2. Method as claimed in claim 1 wherein said ester has the formula 3.Method as claimed in claim 1 wherein said ester has the formula 4.Method as claimed in claim 1 wherein said ester comprises a mixture offrom about 30 to percent by weight of a phosphate ester having theformula:

wherein R is either hydrogen or a C H O(C,H O),, C l-l group in which nis l to 3, m is equal to 2 or 3 and R is a hydrogen atom not more than mminus one times, A is hydrogen or an alkali metal, said ester havand fabout 70 to 39 percent by weight f a 5 ingamolecular weight in the rangeof from about 750 phosphate ester having the formula:

5. Method as claimed in claim 1 wherein said ester is present in thecomposition in an amount of from about 0.3 to 1.5 percent by weight.

6. Method as claimed in claim 1 wherein said ester is present in theform of the sodium salt.

7. Method as claimed in claim 1 wherein said acid is hydrochloric acid.

8. Method as claimed in claim 1 wherein said acid is sulfuric acid.

9. Method as claimed in claim 1 wherein said ester is dissolved in about5 to about percent by weight solution of the mineral acid.

10. Method as claimed in claim 1 wherein said solution is injected downthe injection well bore penetrating said formation under a pressuregreater than the formation pressure and sufficient to create fracturesin the formation. i

11. Method as claimed in claim 1 wherein said solution is injected downthe well bore penetrating said formation under a pressure greater thanformation pres sure but less than the pressure required to createfractures in the formation.

12. A method for the recovery of hydrocarbons from a hydrocarbon-bearingformation containing acid-soluble components and having at least oneinjection well andat least one production well penetrating the saidformation in fluid communication which comprises displacing through theformation a slug of a composition comprising an aqueous acidic solutionof a phosphate ester containing 3 to 30 percent by weight of anon-oxidizing mineral acid and 0.1 to 2.5 percent by weight of saidphosphate ester, said phosphate ester having the general formula:

to about 1730, thereafter injecting a slug of an aqueous drive fluidinto the formation to drive the said composi tion through said formationtoward said production well and recovering hydrocarbons through theproduction well.

13. Method as claimed in claim 12 wherein said phosphate ester has theformula 14. Method as claimed in claim 12 wherein the said phosphateester has the formula 15. Method as claimed in claim 12 wherein saidphosphate ester comprises a mixture of from about 30 to percent byweight ofthe phosphate ester having the formula and from about 70 to 30percent by weight of the phosphate ester of formula 1 16. Method asclaimed in claim 12 wherein said ester is present in the composition ina amount of from about 0.3 to 1.5 percent by weight.

17. Method as claimed in claim 12 wherein said ester is present in theform of the sodium salt.

1. A method for the recovery of hydrocarbons from a hydrocarbon-bearingformation containing acid-soluble components having at least oneinjection well and at least one production well penetrating the saidformation and in fluid communication, which comprises displacing throughthe formation a composition comprising an aqueous acidic solution of aphosphate ester containing 3 to 30 percent by weight of a non-oxidizingmineral acid and 0.1 to 2.5 percent by weight of said phosphate ester,and recovering hydrocarbons through the production well, said phosphateester having the general formula wherein R is either hydrogen or aC13H27O-(C2H4O)n 1-C2H4 group in which n is 1 to 3, m is equal to 2 or 3and R is a hydrogen atom not more than m minus one times, A is hydrogenor an alkali metal, said ester having a molecular weight in the range offrom about 750 to about
 1730. 2. Method as claimed in claim 1 whereinsaid ester has the formula
 3. Method as claimed in claim 1 wherein saidester has the formula
 4. Method as claimed in claim 1 wherein said estercomprises a mixture of from about 30 to 70 percent by weight of aphosphate ester having the formula: and from about 70 to 30 percent byweight of a phosphate ester having the formula:
 5. Method as claimed inclaim 1 wherein said ester is present in the composition in an amount offrom about 0.3 to 1.5 percent by weight.
 6. Method as claimed in claim 1wherein said ester is present in the form of the sodium salt.
 7. Methodas claimed in claim 1 wherein said acid is hydrochloric acid.
 8. Methodas claimed in claim 1 wherein said acid is sulfuric acid.
 9. Method asclaimed in claim 1 wherein said ester is dissolved in about 5 to about20 percent by weight solution of the mineral acid.
 10. Method as claimedin claim 1 wherein said solution is injected down the injection wellbore penetrating said formation under a pressure greater than theformation pressure and sufficient to create fractures in the formation.11. Method as claimed in claim 1 wherein said solution is injected downthe well bore penetrating said formation under a pressure greater thanformation pressure but less than the pressure required to createfractures in the formation.
 12. A method for the recovery ofhydrocarbons from a hydrocarbon-bearing formation containingacid-soluble components and having at least one injection well and atleast one production well penetrating the said formation in fluidcommunication which comprises displacing through the formation a slug ofa compositioN comprising an aqueous acidic solution of a phosphate estercontaining 3 to 30 percent by weight of a non-oxidizing mineral acid and0.1 to 2.5 percent by weight of said phosphate ester, said phosphateester having the general formula: wherein R is either hydrogen or aC13H27O-(C2H4O)n -C2H4 group in which n is 1 to 3, m is equal to 2 or 3and R is a hydrogen atom not more than m minus one times, A is hydrogenor an alkali metal, said ester having a molecular weight in the range offrom about 750 to about 1730, thereafter injecting a slug of an aqueousdrive fluid into the formation to drive the said composition throughsaid formation toward said production well and recovering hydrocarbonsthrough the production well.
 13. Method as claimed in claim 12 whereinsaid phosphate ester has the formula
 14. Method as claimed in claim 12wherein the said phosphate ester has the formula
 15. Method as claimedin claim 12 wherein said phosphate ester comprises a mixture of fromabout 30 to 70 percent by weight of the phosphate ester having theformula and from about 70 to 30 percent by weight of the phosphate esterof formula
 16. Method as claimed in claim 12 wherein said ester ispresent in the composition in a amount of from about 0.3 to 1.5 percentby weight.